Echo suppressor



Oct; 6, 1931. ABRAHAM 1,826,196

ECHO SUPPRESS 0R Filed Sept. 5. 1929 INVENTOR Z. G/flra/mia/z/ ATTORN EY Fatented Oct. 6 1931 o STATES PATENT OFFICE LEONARD G. ABRAHAM, OF TEANECK, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK ECHO SUPPRESSOR Application filed September This invention relates to two-way telephone systems, and more particularly to methods of and means for suppressing echoes in such systems.

The apparatus which is used at present on many types of two-way telephone circuits is disclosed in an article by A. 13. Clark and R. C. Mathes, entitled Echo suppressors for long telephone circuits, appearing in the journal of the American Institute of Electrical Engineers, June 1925, pages 618 et seq. With the arrangements for suppressing echoes which have been devised heretofore, two major difliculties have been present and have not heretofore been substantially overcome. In the first place, when one party has taken control of the transmission circuit, causing the disabling of the one-way path in the opposite direction, an excessive time may be required, once this first party has stopped speaking, for the other party to take control of the circuit so that the currents representing his voice may reach the first party. In the second place, in order that adequate suppression of echoes may be had, it has been necessary to employ arrangements which leave a considerable chance of amutual lock-out, i. e., a situation in which each party causes the disabling of the path which should carry the voice currents of the other party; the result of this situation is, of course, that both parties may speak while neither party hears the voice of the other.

The principal object of this invention is to provide satisfactory suppression of echoes in two-way circuits along with substantial reduction of the breaking time, and lessening of the chance of mutual lock-out.

A further object of the invention is to provide echo suppressor apparatus such that, if two four-wire circuits are in tandem and it is desired to out the system through on a four-wire basis, removing the echo suppressors at the junction point, with advantage to be discussed hereinafter, this may be readily and simply done.

These objects and the various advantages of the invention are accomplished, in general, by placing apparatus at or near each end of the circuit, causing the incoming voice our- 5, 1929. Serial No. 390,540.

rents at either end of the circuit to disable the outgoing path, provided this path is not carrying voice currents which reach the apparatus before the arrival of the incoming currents, and causing the outgoing Voice currents at either end of the circuit to prevent the disabling of the outgoing path by currents in the incoming path of simultaneous or later arrival.

The invention will be clearly understood when the following description of a desirable embodiment thereof and the operation involved is read with reference to the accompanying drawings, in which- Figure 1 shows diagrammatically a fourwire circuit equipped with the novel echo suppressor apparatus of the invention;

Fig. 2. shows a part of the four-wire circuit and indicates a modification of the echo suppressor connections; and

Fig. 3, like Fig. 2, shows a part of the fourwire circuit and indicates an alternative modification of the echo suppressor connections.

Like characters of reference in the three figures of the drawings designate corresponding elements of the circuits.

With reference to Fig. 1 of the drawings, there is disclosed between two two-wire lines L and L a four-wire circuit having a two-wire path LL adapted for transmission from line L to line L and a two-wire path LL adapted for transmission from line L to line L. The usual one-way amplifiers are conventionally represented by A A A and A At the left end of the four-wirecircuit, there is shown the usual hybrid coil H, and an artificial network N balances the two-wire line L; likewise, at the right end of the four-wire circuit, there is disclosed the hybrid coil H and the balancing network l It will be understood, of course, that the four-wire circuit may be quite long1,000 miles or considerably longer-or, in fact, of any desired length; the broken line portions of the lines LL and LL may therefore represent a very long intermediate section of the four-wire circuit, and it is to be understood that the. echo suppressor apparatus shown at the left end of the four-wire circuit is located comparatively near that end of the circuit and the apparatus indicated at the right comparatively near the right end of the circuit and there is no echo suppressor apparatus associated with the intermediate portions of the four-wire p circuit.

Associated with the left end of the four wire circuit are amplifier-detectors AD and AD controlling relays R and R respectively; likewise, at the right end of the four-wire circuit are amplifier-detectors AD and AD controlling relays R and R' respectively. An understanding of the arrangement of the echo suppressor and associated apparatus will be best arrived at by a reading of the following description of the operation of the apparatus; v

Let itbe assumed that the subscriber con nected with the two-wire line at L at the left end of the four-wire circuit starts to talk before the subscriber connected to line L at the right end of the circuit. The speech currents coming over line L, divide at the hybrid coil H, a part of the energy being dissipated in the output of the one-way amplilier A: and the other part being amplified in A and sent on over the path LL. The currents in LL will operate the amplifierdetector AD and the relay R controlled thereby. As clearly indicated in the drawings, the operation of relay R opens a circuit which is adapted to short the transmission path LL, under certain conditions, upon the operation of relay R The speech currents in line LL upon reaching the right end of the four-wire circuit divide at the hybrid coil H between the network N and the twowire line L. The speech currents transmitted toward the network ll enter AD are amplified and rectified and operate relay R The operation of this relay completes a circuit through the armature and contact and places a short upon the transmission line LL, which is adapted, as explained above, to transmit from two-wi re line L at the right to two-wire line L at the left. This short circuit placed upon the line LL causes the suppression of any echoes returned from the line L and the hybrid coil H. The amplifier-detector AD and the relay R at the right end of the four-wire circuit function as do the corresponding elements AD and R at the left end of the circuit. It will be noted in the case of relay R that in addition to winding lV connected in the output of amplifier-detector AD there is a second winding W differentially wound with respect to winding VV This winding 2, it will be noted, is in series with the winding of relay R Accordingly, when relay R is energized, as a result of the operation of amplifier-detector AD the winding lV of relay R is also energized. The action of winding WV: serves to prevent the operation of relay R by any echoes returning from the right end of the four-wire circuit before relay R g has had time to complete its operation. In other words, the contact of relay R is held closed until the contact of relay R is closed, at which time, in addition to the shorting of transmission path LL, there is a shorting of the input of amplifier-detector AD and subsequent currents in LL returning from the hyorid coil H as echoes can have no ellect on the relay R In summary, at the right end of the four-wire circuit, upon the talking of the subscriber connected to line L, a short circuit is placed upon the transi'nission path LL by the operation of relay R and any currents passing over line LL which are of later arrival at the end of the four-wire circuit than the currents in line LL cannot prevent the shorting of line LL at first because of the action of winding YV of r li and subsequently because the full operation of relay R places a short on both the transmission path LL and the input of the aniplitier-detector AD At this point, it should be noted that under certain conditions, in order to prevent the passin of a spurt of echo current over the line LL before relay ll": has fully operated, it may necessary to insert a dela circuit, as indicated, or the equivalent between the hybrid coil H and the input to amplifier-detector jrD If the subscrib r connected to line L begins to talk after the subscriber at the distant end of the circuit, the formers speech current will be suppressed until the sub scriberconnected to line L has stopped talking long enough to permit the release of relay R As soon as this relay has released the voice currents or' 'inating with the subscriber on line L will cause the operation of'relay' 1' and the opening of the circuit adapted to short the transmission path LL and if the distant speaker should start to speak again, causing the operation of amplifier-detector AD and the relay R the closing of the contactof this relay Wlll have no effect sincethe shorting circuit is opened at relay 11' The voice currents passing over line LL will reach the left end of the fourwire circuit, operate relay R and close the shorting circuit on transmission path LL; thus, the return of echoes from the left end of the four-wire circuit to the right end is prcventet. As is the case at the right end of the circuit, the winding N of relay R is connected in scaes with the winding of relay It, is dilierentiz My wound with respect to the winding \V and, accordingly, serves to hold the contact of relay It; closed until relay R: has had time to complete its operation and disable the path LL and the amplifier-detector AD 'From the above discussion, it will be understood that if the near talker with respect to either end of the four-wire circuit begins to speak before the distant talker, the out mutual lock-out; there will merely be the situation, in the event of speech beginning nearly simultaneously at the two ends, of both subscribers talking and each subscriber hearing the other. Any echoes in the circuit will do no harm as both subscribers will be talking and not listening in such a case.

This protection from mutual lock-out is one of the major advantages of the applicants arrangement.

In case two or more circuits equipped with this t e of a aaratus are connected in tandem it is possible for lockouts to occur between the echo suppressors at the ]unction points on the venous circuits. The chance that such lock-outs will occur, however, is much smaller than would be the case 1f the circuits were equipped with the type of apparatus now in use. This possibility of lockout, moreover, may be eliminated by removi ing the echo suppressors from the circuits at the unction points.

Due to the position of the echo suppressor apparatus at or near the ends of the four wire circuit and due to the novel arrangement of the echo suppressor apparatus, the breaking time, i. e., the time during which one talker must pause in order to permit the other to take control of the circuit, can be materially reduced. A comparison of applicants arrangement with the arrangement disclosed in the above identified article by Clark and Mathes will make this clear.

It will be noted that in the arrangement of Fig. l, the amplifier-detectors A13 and AD are connected between the hybrid coil H and the network N and between the hybrid coil H and network N, respectively. This point of connection is not essential, though in many cases it will be desirable. In Fig. 2 of the drawings, the amplifier-detector AD at the right end of the four-wire circuit is shown connected between the one-way amplifier A and the hybrid coil H; in Fig. 3, the amplifier-detector AD is shown connected to the transmission path on the input side of the one-way amplifier A It will be understood, of course, that corresponding connections may be made at the distant end of the four-wire circuit. In the case of Fig. 2 and Fig. 3, if the distance from the end of the circuit to the echo suppressor apparatus is great enough, no artificial delay will be necessar 'v llhile the invention has been disclosed in certain specific embodiments which are deemed desirable, it is to be understood that the true scope of the invention is not limited thereby but is determined by the appended claims.

What is claimed is:

1. In a two-way telephone circuit including paths adapted for transmission in opposite directions, apparatus associated with each of two distant points at or relatively near the respective ends of the circuit, said apparatus comprising means operable by voice currents in the receiving path for disabling the transmitting path, means operable by voice currents in the transmitting path for preventing the disabling of said path, and circuitarrangements connecting said first named means with said second named means in such a manner as to oppose the operation of the latter while the former is in process of operation.

2. In a two-way telephone circuit including paths adapted for transmission in opposite directions, apparatus relatively near each end of the circuit, comprising means whereby the outgoing path will normally be disabled in response to voice currents in the incoming path and means whereby currents in the outgoing path will prevent the disabling of said path by subsequently arriving currents in the incoming path.

3. In a two-way telephone circuit including paths adapted for transmission in opposite directions, apparatus relatively near each end of the circuit, comprising means whereby the outgoing path will normally be disabled in response to voice currents in the incoming path and means whereby currents in the outgoing path will prevent the disabling of said path by subsequently arriving currents in the incoming path without I disabling the incoming path.

4. In a two-way telephone circuit including paths adapted for transmission in opposite directions, apparatus relatively near each end of the circuit, comprising means whereby the outgoing path will normally be disabled in response to voice currents in the incoming path, means whereby currents in the outgoing path will prevent the disabling of said path by subsequently arriving currents in the incoming path, and means whereby later fiowing currents in the outgoing path are prevented from interrupting the disabling of said path by earlier flowing currents in the incoming path.

5. In a two-way telephone circuit having paths adapted for transmission in opposite directions,echo suppressor apparatus associated with each end of the circuit and so arranged that it the near talker, with respect to either end, begins to speak before the distant talker, the outgoing path will be clear as long as the outgoing speech continues and the incoming path will be disabled, whereas if both talkers begin to speak at approximately the same time, both paths will be clear.

6. In a two-Way telephone circuit including paths adapted for transmission in opposite directions, echo suppressor and associated apparatus relatively near eachend of the circuit, said apparatus comprising means responsive to voice currents in the incoming path for closing a circuit designed to short circuit the outgoing path, means responsive to voice currents in the outgoing path for preventing the closure of said shorting circuit, thereby preventing the disabling of the outgoing path, and means responsive to current fiOW in the incoming path for preventingthe operation by currents of later arrival of said means for preventing the closure of the shorting circuit.

7. In a two-Way telephone circuit including paths adapted for transmission in opposite directions, echo suppressor and associated apparatus relatively near each end of the circuit, said apparatus comprising means responsive to voice currents in the incoming path for closing a circuit designed to short circuit the outgoing pat-l means responsive to voice currents in the outgoing path for opening said shorting circuit, thereby preventing the disabling f the outgoing path, and means responsive to current flow inthe incoming path for preventing the operation by currents of later arrival of said means for opening the shorting circuit.

8. In a two-way telephone circuit including paths adapted for transmission in opposite directions, echo suppressor and associated apparatus relatively near each end of the circuit, said apparatus comprising means responsive to voice currents in the incoming path for closing a circuit designed to short the outgoing path, means responsive to voice currents in the outgoing path of earlierarrival than currents floxving in the incoming path for preventing the closure of said shorting circuit, thereby preventing the disabling of the outgoing path, and means responsive to current flow in the incoming path for preventing the operation by currents of later arrival of said means for preventing the closure of the shorting circuit.

9. In a two-Way telephone circuit having paths adapted for-transmission in opposite directions, the method of suppressing echoes which consists in causing the incoming voice currents near either end of the circuit'to disable the outgoing path, provided said path is not carrying voice currents of earlier arrival, and causing the outgoing voice currents near either end of the circuit to pre vent the disabling of the outgoing path by currents in the incoming path of simultaneous or later arrival.

10. In a tivoqvay telephone circuit having paths adapted for transmission in opposite directions, the method of suppressing-echoes 

